The U.S. imposed a blockade on sanctioned oil tankers tied to Venezuela. How is that working out?

Questioned over the weekend about how the U.S. government will take over governing Venezuela, Secretary of State Marco Rubio instead shifted the focus to the U.S. blockade of sanctioned oil tankers. CBC News looks at the origins of the blockade, what it entails, whether it's considered legal and just how effective it is.